All self-powered firearms have a natural cyclic rate. The natural cyclic rate of each firearm is a function of its design so the natural cyclic rate is merely an outcome of the design. Unfortunately, the natural cyclic rate of a firearm may not be the optimum cyclic rate for the target engagement scenarios most commonly encountered. Generally speaking, the natural cyclic rate of firearms intended for antipersonnel use is far higher than would be optimum. The cyclic rate of a firearm is usually expressed as the number of Shots Per Minute (spm) that the firearm would discharge when fired in the fully automatic mode, although in actual practice firearms are seldom fired continuously for one minute.
Most shoulder-fired fully automatic firearms such as the M16 family of rifles and the M4 family of carbines have such high natural cyclic rates of fire that the rapidly delivered recoil impulses to the shooter cause the weapon to move off target uncontrollably. This not only reduces hit probability, but wastes ammunition, overheats and rapidly wears out mechanical aspects such as barrels, can cause a serious safety hazard to fellow soldiers and bystanders, and reduces “trigger time” for the available ammunition. In most cases this pervasive uncontrollability is simply tolerated and/or somewhat mitigated by training soldiers to fire short bursts or by incorporating burst limiters within the firearm mechanism. On the other hand, a way of actually improving controllability is to reduce the cyclic rate.
The M16/M4 families of firearms possess a natural cyclic rate of fire of 700 to 950 spm. When fired from the offhand position in fully automatic fire by experienced (right handed) shooters, controllability testing has shown that at 100 yards the second projectile of a burst strikes approximately one foot to the right and above the impact of the first projectile, and the third projectile strikes approximately two feet to the right and above the second projectile (three feet off target). Furthermore it takes until about the seventh round of a burst before the shooter can force the shots back approximately onto target. Then when the trigger is released, the firearm plunges down and to the left (down and to the right for a left handed shooter). This makes target reacquisition time consuming/difficult.
The M4 family of carbines is physically lighter than the M16 family of rifles, making the M4 even less controllable. The uncontrollability of the M4 Carbine (which is typical of current military rifles and carbines) in full automatic fire also contributes to wastage of ammunition, excess barrel heating, etc. Rifles having heavier recoil than the 5.56 mm NATO Cartridge (such as those chambered for 7.62 mm NATO) greatly exacerbate the controllability problem.
In order to ameliorate the waste of ammunition, the M4 and some other variants of the M16 are equipped with a three round burst limiter. Three round burst limiters do not so much provide increased hit probability, but rather provide more trigger time/pulls per magazine.
Some rate reducers lower the natural cyclic rate by slowing the average velocity of the recoiling parts through the use of hydraulic buffering. The amount of rate reduction achievable using hydraulic buffers is limited because the recoiling parts themselves are slowed, and the firearm cannot function at all below a certain operating mechanism velocity. This is because the minimum amount of momentum required to carry the recoiling parts through the cycle of functioning is lost. The term “recoiling parts” is applied to those parts of the firearm mechanism (such as the bolt, bolt carrier, etc.) that travel from battery to full recoil (and back) during the cycle of functioning. The term applies to those parts whether the parts are actually moving in recoil or in counter recoil, toward battery.
The U.S. military, as well as civilian industry, have developed several hydraulically based rate reducing mechanisms for the M16/M4 family of weapons; however, hydraulic rate control mechanisms do not achieve an effective reduction in cyclic rate. In these systems the bolt carrier is brought more slowly to a stop in recoil. While this slowing results in somewhat reducing the cyclic rate, it also results in reduced functional reliability because energy is removed that is required for reliably cycling the mechanism. Additionally, hydraulic buffers react unfavorably to extreme hot and cold environments; delivering less rate reduction in high temperature environments and being sluggish at cold temperatures. The largest disadvantage, however, with hydraulic systems is their inherent inability to adequately reduce the cyclic rate sufficiently to substantially increase hit probability.